This invention relates generally to the field of fluid flow sensors. More specifically, it relates to improvements in fluid flow sensors of the variable orifice or variable obstruction area type, that are commonly used, in conjunction with a pressure transducer, to generate a differential pressure signal from fluid (especially gas) flow in a conduit, wherein the value of the differential pressure signal is correlated with a flow rate value. The flow rate value, in turn, may be integrated over time to yield a volumetric value.
Flow sensors of the class described above have become commonly used in medical applications, particularly for measuring the flow rate of respiratory gas in medical ventilators. Specific examples of such flow sensors are found in the following U.S. Pat. Nos. 4,989,456--Stupecky; 4,993,269--Guillaume et al.; and 5,038,621--Stupecky. The variable orifice flow sensors exemplified by these patents employ a hinged obstruction or flapper that is mounted within a flow orifice of known area. The flapper is mounted so that the portion of the total area of the orifice that it opens to fluid flow is proportional to the flow rate through the orifice. The pressure drop across the orifice is proportional to the open area through the orifice. Thus, the differential pressure across the orifice is directly related to the flow rate through the orifice. This pressure differential is sensed by pressure ports upstream and downstream from the orifice. The sensed upstream and downstream pressures are directed to a differential pressure transducer, which generates an analog electrical signal having a value representing the differential pressure value. The analog signal is digitized and input to a microprocessor that is programmed to compute a value representing the instantaneous volumetric fluid flow rate through the orifice.
While flow sensors of the type described above have exhibited acceptable levels of accuracy and reliability, further improvements have been sought. Specifically, the use of such flow sensors in respiratory therapy equipment, particularly medical ventilators, has led to their fabrication from materials, such as stainless steel, that can be sterilized in autoclaves. A flapper made of stainless steel, however, is susceptible to fatigue and failure (especially at its hinged attachment point) due to repeated deflections over long periods of use, and due to overstress in response to high flow rates. This problem could be overcome by strengthening the hinged attachment part of the flapper and by increasing the area of the flapper so that it is not overstressed at high flow rates, but this solution would result in a loss of resolution at low flow rates.
Furthermore, it has been found that the use of stainless steel flappers has, in itself, resulted in the loss of low-end resolution. This is because, during the fabrication process, the stainless steel sheet takes a "set" that results in a flapper that deviates somewhat from a true planar configuration. In other words, the flapper is often slightly curved, so that, when it is installed in the orifice, there is a gap between the flapper's peripheral edge and the annular surface that defines the orifice. This results in a non-zero orifice area at zero flow rate, which, in turn, results in the loss of resolution at low flow rates. Furthermore, to avoid deterioration in accuracy due to exposure to the high temperatures of an autoclave, the flapper is advantageously annealed. The annealing process, however, softens the stainless steel, and the resultant loss of stiffness or rigidity causes the flapper to lose a measurable degree of responsiveness, most noticeable at low flow rates. Thus, the annealing process, while addressing one cause of inaccuracy (high temperature exposure), introduces another cause (loss of flapper rigidity).
Thus, there has been a need for a variable orifice flow sensor with a metal (preferably stainless steel) flapper that is capable of withstanding repeated deflections over a long period of use without fatigue or failure, even with repeated exposure to high flow rates. There has been a further need for such a flow sensor that also yields good low end sensitivity and resolution. There has been a still further need for such a flow sensor that is capable of withstanding repeated autoclaving without deterioration of accuracy over time, and without sacrificing responsiveness.